Many mass analysers that provide accurate mass measurements use electrostatic fields that are generated by high voltage power supplies. In some applications, for example liquid chromatography mass spectrometry (LC/MS) with ionisation at atmospheric pressure, the ionisation efficiency of particles for analysis may be optimal at different polarities. In such cases, analysis of all ions demands that the polarity of the electrostatic field of the mass analyser be switched. For accurate mass analysis, it is desirable that the stability of the electrostatic field be maximised.
Some existing technologies provide both positive and negative potentials using one or more power supplies having the same polarity. Polarity switching can then be achieved by powering down the entire high voltage network, switching relays to invert the polarity of the power supply output, and powering up the high voltage network again. Pulser wiring or triggering may also require adjustment. Moreover, a different feedback resistor chain may be used for voltage regulation, between different polarities. Heating and stabilisation of the entire network, once it has been powered up may take a number of hours. During this time, when the potentials provided for generating the electrostatic field may be unstable, accuracy of the mass analyser is poor for these reasons. WO-2004/107388 and WO-2008/081334 also illustrate schemes for injection of ions into a mass analyser that require stable and accurate potentials.
A high-voltage power supply with improved switching speed is described in WO-A-2007/029327. This is designed for powering a conversion dynode. Two power supplies are used, each providing a voltage with an opposite polarity with respect to the other. The polarity of the power supply output is changed by powering down the supply providing the unwanted polarity and regulating the other power supply output at the desired level. The polarity switching-speed is therefore improved by sacrificing accuracy of the output voltage.